Conventionally, a perfluorinated sulfonic acid membrane (ex. Nafion (trademark) made by DuPont) is used as an electrolyte membrane, which determines the performance of the polymer electrolyte fuel cell. However, since it is expensive, hydrocarbon polymer based membrane has been studied in recent years. This kind of membrane has a structure of a sulfonic acid group directly bonded to an aromatic ring of the polymer. However, the performance of the membrane is deteriorated due to desulfonation reaction occurring gradually, as disclosed in Non-patent Reference 1, when it is used for a long period of time under acidic condition at 100 degrees Celsius or higher. Such a mechanism emanates from an electrophilic substitution reaction occurring due to proton attacking to the aromatic ring. Therefore, the development of a method of directly bonding the sulfonic acid group to a different substrate from the aromatic ring has been desired.
For such a purpose, a chemically modified fullerene made by directly bonding the sulfonic acid group to fullerene is disclosed in Patent Reference 1. However, it has a problem that a target substance cannot be obtained because dimethylformamide used as a reaction solvent is also bonded when a sulfonation reaction occurs. To solve this problem, Patent References 2 and 3 disclose methods of bonding the sulfonic acid group to fullerene via hydrocarbon or fluorinated spacer molecules. However, these methods have shortcomings such that they require a complex production system and that higher ion-exchange capacity cannot be provided.    Patent Reference 1: Japanese Unexamined Patent Application Publication No.2002-326984    Patent Reference 2: Japanese Unexamined Patent Application Publication No.2005-93417    Patent Reference 3: Japanese Unexamined Patent Application Publication No.2005-68124    Non-patent Reference 1: ‘Development of PEFC electrolyte membrane’, page 30, supervised by Kyoji Kimoto and published on December in 2005 by CMC Publishing Co., Ltd.
The inventor of this application has succeeded in directly bonding the sulfonic acid group to fullerene core without bonding of a reaction solvent by using K2SO3 as a sulfonation reagent and dimethylacetamide plus water as a specific reaction solvent. Meanwhile, phosphonated fullerene is generally disclosed in Patent Reference 2, however, a detailed preparing method is not disclosed, and the chemical structure of directly bonded type is not specified. The inventor of this application has used LiPO(OEt)2 as a phosphonation reagent and dioxane as a preferable reaction solvent to successfully provide phosphonated fullerene of a direct bonding type. Since the phosphonic acid group is bivalent, it is useful to increase ion-exchange capacity and proton conductivity. Further, the chemically modified fullerene bearing either the sulfonic or phosphonic acid group is soluble in water. However, it should be water-insoluble when used as a proton conducting membrane electrolyte in a fuel cell. The phosphonic acid group is useful because it can be metal-bridged using polyvalent metal ions such as calcium or platinum ions to make the chemically modified fullerene water-insoluble.